1. Field of the Invention
The field of invention relates to the production of mixed xylenes. More specifically, the field relates to the production of mixed xylenes using heavy reformate.
2. Description of the Related Art
Petrochemical refiners are facing issues with the utilization of heavy reformate streams. Environmental regulations are limiting the amount of aromatics (C6+) content in gasoline fuel. Gasoline refiners and blenders traditionally use C6+ aromatics, which include the C6-8 BTEX components (Benzene, Toluene, Ethylbenzene and mixed Xylenes) to improve octane ratings and anti-knocking attributes of motor fuels.
Although motor fuel use is a major consumer of BTEX components, each of the BTEX products has alternative markets for their use besides motor fuels. Benzene is a widely used precursor for many chemical and solvation processes. Toluene and ethylbenzene are also reactants and precursors in chemical and polymerization processes. The three isomers of xylene (C8H10)—para-xylene (p-xylene), meta-xylene (m-xylene) and ortho-xylene ( o-xylene)—are all petrochemical feedstocks of value.
“Mixed xylenes” is a mixture of all three xylene isomers. P-xylene is a feed for polyester, a polymer with significant and growing global demand. O-xylene is the feedstock for phthalic anhydride manufacturing, which, in turn, feeds alkyd resin and urethane production. M-xylene has a lower commercial demand than the other two xylenes and makes isophthalic acid and plasticizers. When mixed xylenes form during catalytic reformation, meta-xylene forms in greater amounts than para-xylene and ortho-xylene because of thermodynamics. Market demand, however, currently favors p-xylene followed distantly by o-xylene.
Reformate, which is the bottoms product from naphtha catalytic reforming, and pyrolysis gasoline (“pygas”), a byproduct of ethylene cracking, are the usual sources for these compounds. Extracting the BTEX components from reformate and pygas leaves a heavy reformate, which comprises mainly C9+ alkyl aromatic compounds (pygas may also contain diolefins). Heavy reformate can also form from running hydrogen production process at a greater severity (that is, temperature) to produce additional hydrogen alkanes and olefins from the same naphtha feedstock.
The heavy reformate as a chemical stream in and of itself has little direct commercial value. Any ability to blend away C9+ alkyl aromatics into gasoline pools is shrinking rapidly.
Toluene is not usually found in heavy reformate as it has been previously recovered or the heavy reformate forms under severe processing conditions such that light aromatics like toluene do not form. Often petrochemical refiners that use transalkylation to convert tri-methyl benzenes found in heavy reformate into xylenes add previously extracted toluene back into the heavy reformate stream. This is a waste of processing energy and unnecessarily increases capital expenditure.